Erm I hope you guys don't mind the GIGANTIC IMAGES
Pretty mainstream I know, but I haven't read that many manga yet. So bear with me.
I have to admit that Zetman- and Berserk's art are totally awesome though!

Eyeshield 21, Murata Yusuke: This guy is godlike with colors, and this was only when he was at chapter 155 of Eyeshield 21 Sorry Oh Great!, but this guy definitely surpassed you! After all, he said it himself that he looked up to Oh Great!, the author of Air Gear and Tenjou Tenge But this guy right here has become my favorite artist!

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Naruto, Masashi Kishimoto: I've always liked the way he managed to draw faces, with just a few simple lines and circles!

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Fanservice

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Death Note,Takeshi Obata: Another very awesome artist, probably one of the most realistic art styles I've ever seen!

Bleach, Tite Kubo: While his writing is not the best, he still managed to draw very well and detailed with a rather semi realistic art style.

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One Piece, Eichiro Oda: You gotta love how this guy managed to make this manga with such a cartoony art style, the best selling manga in the world! I really shouldn't have judged it on its art, when I first saw it! After all, it's the story that truly mattered here!

Wow, Image wall made my mobile browsing cry! Anywhoo, I managed to find good sample pics online. Made sure to get actual in story art, not just the cover art though in the case of Shaman warrior, is done by manga artist as well and is phenomenal.

Top three for me.

3. Hitoshi Iwaaki with "parasyte". Simpler art, but is so unique and never have I seen it sucessfully emulated. Plus the way he can generate emotion from his images is astounding!

2. Park Joong-Ki with his series "Shaman Warrior"

1. Hiroshi Hirata whose art and Kangi is so well done that when they translated Satsuma Gishiden, they kept his original hand kangi in and subbed it in the manga.

I suppose I shouldn't be surprised that Berserk isn't mentioned too often, it is kind of for that certain niche of people, and mature as well.

That said; Berserk!

It's not like almost every manga named previously- it's good art is constant! Constant in that the artist doesn't just throw in one or two pages every couple of volumes to show off or emphasize something. And it gets better as the manga progresses.

Note, though, that good drawings do not make a good manga. Take One's One-Punch man, for instance (the original, not the remake). His drawings are.. well.. laughable in comparison to something like Berserk, but it's one of my favorite mangas.